1. Field of the Invention
This invention relates to methods for transferring a uniform phosphor layer on an article, and, more particularly, to a method of transferring a uniform phosphor layer that converts LED light wavelengths on an article and a light-emitting structure fabricated by the method.
2. Description of Related Art
Phosphor materials are widely applied to LED packages that include blue pump LEDs and green or red phosphors and emit white light (e.g., a mixture of blue light emitted from the blue pump LEDs with green or red light converted from the blue light by the phosphor materials). Conventional methods for depositing phosphor materials on blue LED chip or package assembly include:
Slurry method: phosphor powders are distributed in silicon, epoxy resin or solvent filling material, to form a phosphor mixture, and the phosphor mixture is applied to a surface of an LED or a package lens material by spraying coating or immersing coating techniques.
Electrophoretic deposition (EPD): phosphor powders are distributed in electrochemistry solution and deposited on an LED wafer through a bias voltage spanning across the LED wafer and the electrochemistry solution. The conventional methods encounter a problem that the surface of the LED or the interior of the LED package does not have a uniform thickness. The slurry method forms a layer of particles that does not have a uniform thickness. As a result, the LED does not have uniform light color points and the light converted by the phosphors has poor color uniformity. Moreover, the conventional methods are difficult to be applied to an uneven surface and to form a uniform layer of phosphors. It is a real challenge to use the conventional methods to satisfy lighting application demands.
In the conventional coating process, such as the slurry method that applies the phosphor mixture to an LED chip, wafer or package, the phosphor powders are mixed with silicon, and the mixture is applied to the LED chip, wafer or package by “glob” dispensing, spin-coating, injection, Electrophoretic deposition or molding methods. The subsequent wire-bonding process thus faces a great challenge, because it is hard to pattern the silicon material that is cured, which cannot applied with standard developer and photoresist. Therefore, the phosphor mixture are generally coated or deposited following the packaging stage such as the wire-bonding process.
It is also known that a remote phosphor technique, when applying phosphor silicon to an uneven package surface of an LED, faces a problem regarding the uniformity of phosphor coating, since the phosphor-silicon mixture has a viscosity greater than that of the cured LED encapsulant, and thus has a greater curvature, i.e., the layer of phosphors having a central region thicker than an outer region. In the application of the remote phosphor technique, forming a uniform phosphor coating layer on second-order optical elements of an LED also faces the same challenges.
The conventional methods still suffer the problems that the distribution capacity of phosphors cannot be controlled, the LED does not have consistent light color points, and the light converted from the phosphors has poor color uniformity. Therefore, how to provide a method for transferring a uniform phosphor layer on an article and a light-emitting structure thus fabricated is becoming one of the most popular issues in the art.